Facts About the Kuiper Belt
- The Kuiper belt contains millions of icy objects. They come in all sizes, from small lumps of ice to large objects around 100 km in diameter or more.
- The Kuiper belt was named after astronomer Gerard Kuiper. Sometimes, it is also called the Edgeworth-Kuiper belt after Kenneth Edgeworth.
- Around 35,000 Kuiper belt objects are estimated to be larger than 100 km in diameter. This is several hundred times the number and mass of objects found in the asteroid belt.
- There may be as many as 100 million small and faint objects in the Kuiper belt, with diameters of 20 km or less. However, a Hubble Space Telescope follow-up observation was not able to confirm it.
- Three officially recognized dwarf planets are located in the Kuiper belt. These are Pluto, Haumea, and Makemake.
- The largest object in the Kuiper belt is the dwarf planet, Pluto. Its status as part of the belt is what caused the planet to be reclassified as a “dwarf planet” in 2006.
- Eris, which is more massive than Pluto, is located in the scattered disk. Though, it is believed that it was originally found in the Kuiper belt.
- Neptune’s moon Triton is also larger than Pluto. It is believed to have been captured from the Kuiper belt due to gravitational encounters.
- In 1983, NASA’s Pioneer 10 was the first spacecraft to enter the Kuiper belt. Before that, between 1972 and 1973, it was also the first to traverse the asteroid belt.
- NASA’s New Horizons mission is the first mission dedicated to the Kuiper belt. It flew past Pluto in July 2015. It also surveyed Pluto, Charon, and the other moons. New Horizons is now continuing its course to the other objects in the belt and beyond
The Kuiper Belt
The Kuiper belt is a region of space. It is located in the outer solar system beyond the orbit of Neptune. This big swarm of objects form a doughnut-shaped ring. Like the asteroid belt, it is populated by leftover objects from the formation of the solar system. Though there are similarities, these two regions are also very different.
|Asteroid Belt||Kuiper Belt|
|Location||Between the orbits of Mars and Jupiter||Beyond Neptune’s orbit|
|Distance||Between 2.2 and 3.2 AU from the Sun||Between 30 and 55 AU from the Sun|
|Composition||Rocky objects||Icy objects|
|Some famous objects||Ceres, Vesta, Pallas, Hygiea||Pluto, Haumea, Makemake, Charon|
While the asteroid belt is located in the inner solar system, the Kuiper belt lies beyond the orbit of Neptune. It is 20 times larger than the asteroid belt and 20 to 200 times more massive.
The Kuiper belt is a very cold and dark region, stretching between 30 and 55 AU from the Sun. One AU is the distance between the Earth and the Sun or roughly 93 million miles. Now, take that distance, but 30 to 50 times more. That is how far the Kuiper belt is!
Did You Know?
Some of the big moons in the solar system are thought to be from the Kuiper belt region. Examples are Triton and Phoebe which are likely captured by Neptune and Saturn, respectively.
The asteroid belt is mostly composed of rocky objects. Alternatively, the Kuiper belt is mostly populated with icy objects and comets. In the outer stretches of the Kuiper belt lies the scattered disk. The main part of the Kuiper belt is dynamically stable but the scattered disk is dynamically active.
Objects in the scattered disk have very elliptical orbits that are also very tilted. In fact, this zone is believed to be the source of short-period comets. Comets of this type orbit the Sun in less than 200 years. Halley’s Comet is an example of a short-period comet.
At the Edge of the Solar System
So, is the Kuiper belt the end of the solar system? No. The asteroid belt and the Kuiper belt are different from the hypothetical Oort cloud. Far beyond the Kuiper belt, there is a spherical shell called the Oort cloud.
Just like the Kuiper belt, the Oort cloud is also made of icy space debris. However, it does not form a disk, but a shell. We can think of it as a big bubble that surrounds everything in the solar system.
To put it simply, our solar system is structured this way:
We have not directly observed the Oort cloud because of the great distance. Still, the strongest evidence of its existence is that it must be the source of many long-period comets beyond Pluto. An example of this is the Hale-Bopp Comet which takes around 2,520 years to orbit the Sun. Also, the large and distant planetoid, Sedna, is believed by some astronomers to be from the inner Oort cloud.
The Kuiper Belt Through Time
The main part of the Kuiper Belt begins at Neptune’s orbit. Credit: NASA
The existence of the Kuiper belt was hypothesized for a long time. It all started in 1930 when Pluto was discovered. Scientists believe that Pluto might not be alone—and they are right.
We often hear the names Kenneth Edgeworth and Gerard Kuiper when talking about the Kuiper belt. They are not the discoverers of the said circumstellar disk. However, their ideas about it were some of the most famous.
Theoretical astronomer Kenneth Edgeworth published a paper in 1943. He speculated on the presence of small bodies beyond Neptune which are remnants of the solar nebula. In 1951, Dutch astronomer Gerard Kuiper also had a similar hypothesis.
Many other theories have formed over time. Though he did not predict its existence, the belt was named after Gerard Kuiper. Sometimes, it is also called the Edgeworth-Kuiper belt.
American astronomer Fred Whipple also contributed greatly to the understanding of the Kuiper belt. His “dirty snowball” hypothesis also speculated on the presence of this region which he called a “comet belt.”
Did You Know?
The idea that objects exist together with Pluto is has been there for a long time. Many people have different theories about it. Because of that, no one is really credited for originally proposing this idea.
Origin and Formation
Astronomers call the small objects in the solar system planetesimals. These space debris have been around since the formation of the solar system. Most of them pulled together to form the Sun. Those that remained are what formed the planets.
While some planets were formed, some may not have come together successfully. This is the case for the Kuiper belt region. The icy objects in the region most likely swarmed together to form a planet. However, Neptune’s gravity stirred them up and disrupted the process.
The same is true about the asteroid belt. Had Jupiter not been there, a different planet may have formed in the region as well. Because of Jupiter and Neptune, objects in the asteroid belt and the Kuiper belt were not able to coalesce. Still, these smaller objects remain in orbit around the Sun.
According to the Nice model, the outer planets moved inward and outward in the early solar system. Much like a celestial dance, right?
An orbital resonance between Jupiter and Saturn disturbed the orbits of Uranus and Neptune. Ultimately, the ice giants were sent further from the Sun. As they migrated, they destabilized the smaller bodies around them. These planetesimals were scattered inward while some were thrown further. Some of the planetesimals that moved inward were captured by the giant planets and became their moons.
The Kuiper belt has lost most of its original material because of the migration of gas giants. It is estimated that the lost material is roughly 7 to 10 times the Earth’s mass.
Discovering Kuiper Belt Objects (KBOs)
Much about the dark and distant Kuiper belt region was a mystery, until 1992. That was when the second KBO after Pluto was discovered.
Two scientists, David Jewitt and Jane Luu, believed that the universe is not empty. Because of that, they started their search for astronomical objects beyond Neptune’s orbit. They started scanning the heavens in 1987 using telescopes from different observatories.
The pair conducted their study with a blink comparator, just like Clyde Tombaugh when he discovered Pluto. They did their research at the Kitt Peak National Observatory (KPNO) in Arizona and the Cerro Tololo Inter-American Observatory (CTIO) in Chile. Later, they moved their studies to the University of Hawaii.
Did You Know?
Centaurs are a different type of objects that orbit the Sun between Jupiter and Neptune. They are likely from the Kuiper belt but were pushed inward by Neptune’s gravity.
In 1992, Jewitt and Luu finally found a candidate KBO that was 44 AU from the Sun. At that distance, this reddish object was even more distant than Pluto (39.5 AU). The two discoverers wanted to call this object “Smiley.” However, this name was already taken by the asteroid 1613 Smiley. Instead, it was provisionally designated (15760) 1992 QB1. In 2018, it was given the permanent name 15760 Albion.
The legacy of uncovering Albion led to the discovery of many other trans-Neptunian objects (TNOs). In fact, there were more than 2,000 known KBOs in 2018.
The objects in the Kuiper belt can be classified into two large classes: classical and resonant. They are grouped based on Neptune’s influence on their orbits.
Classical KBOs are also called “cubewanos.” They lie at an average distance of 40 to 50 AU from the Sun. Compared to other objects in the belt, cubewanos have relatively circular orbits that are not very tilted. Also, their orbits are not controlled by Neptune’s influence.
There are two types of classical KBOS: cold and hot.
Cold Classical KBOs
Cold classical KBOs have orbits with low eccentricity and inclination. They are relatively more circular and are not tilted to the plane of the planets. Because their orbits are not elongated, they mostly remain at the same distance from the Sun. Most of the classical bodies belong to this population.
Hot Classical KBOs
The orbits of hot classical KBOs are more elliptical and tilted. Since their orbits are elongated, there will be times when they will be closer to the Sun. In some parts of their orbits, they will also be farther.
Hot classical KBOs had interactions with Neptune’s gravity in the past which made their orbits tilted and elongated. In contrast, the cold classical KBOs never came close to the giant planet. Because of that, their orbits remain unperturbed.
Distribution of trans-Neptunian objects. Objects occupying the stronger resonances are in red.
Resonant KBOs have orbits that are in a stable and repeating pattern with Neptune. That means, for every specific number of orbits they complete, Neptune also completes a specific number. This is more like a ratio.
For example, Pluto is in a 2:3 resonance with Neptune. That means it completes 2 orbits around the Sun for every 3 orbits that Neptune makes. Aside from that, there are also other resonances like 1:1, 1:2, and 2:5.
These numbers can also be written the other way, like 3:2 for Neptune and Pluto. Since the trans-Neptunian objects have longer orbital periods than Neptune, the smaller number corresponds to them.
Just like Pluto, many objects are also in the same 2:3 resonance with Neptune. Because of that, the category Plutinos was made. Most resonant objects are in this category. As of February 2020, there are already 383 confirmed plutinos.
Prominent Kuiper Belt Objects
The discovery of one KBO can lead to a better understanding of the others. There are so many of these objects but below are some of the famous ones.
Pluto was once the ninth planet in our solar system. In 2006, it was reclassified as a dwarf planet since it has not “cleared the neighborhood” around its orbit. It was the first KBO to be discovered.
Pluto is the largest dwarf planet, but only second to Eris when it comes to mass. It lies around 40 AU from the Sun. A year in this distant world takes roughly 248 Earth years. However, a day there is only about 6 hours long.
Pluto is 5.5 times smaller than the Earth. It is also smaller than our Moon. One of its most distinguishing features is the heart-shaped glacier called Tombaugh Regio. This feature is roughly the size of Texas and Oklahoma.
Pluto has blue skies, high mountains, and a thin atmosphere. It also snows there, but instead of white, they are red in color. This dwarf has 5 moons: Charon, Hydra, Kerberos, Nix, and Styx.
Makemake is another dwarf planet in the Kuiper belt. It is located about 45 AU from the Sun. At that distance, it takes sunlight 6 hours and 20 minutes to reach it. A day on this cold world is about the same length as the Earth’s but a year there takes 305 Earth years.
Makemake was discovered in 2005. Together with Eris, it is one of the reasons for the creation of the dwarf planet category. After Pluto, it is the second-brightest KBO that we can see on Earth. Also, it has a provisional moon nicknamed MK2.
Haumea is a uniquely shaped dwarf planet in the Kuiper belt. It is about the same size as Pluto. Since it rotates very fast, its shape has become distorted and it now looks like an egg. Its discovery was rather controversial because two teams claimed credit for discovering it.
Haumea lies 43 AU from the Sun. It rotates every 4 hours, making it one of the fastest-spinning large bodies in the solar system. A year in Haumea is equivalent to about 283 years on Earth. It has two known moons, Hi’iaka and Namaka.
Quaoar is a planetoid in the Kuiper belt. Discovered in 2002, this object is roughly 1,121 km (697 mi) across. At that time, it was the largest object detected since Pluto’s discovery. Because of that, it was nicknamed “Object X” as a reference to the theoretical Planet X. Years later, objects bigger than Quaoar were discovered like Eris and Haumea.
Quaoar follows a nearly circular orbit and it is not in resonance with Neptune. In 2007, a moon around it was discovered. This natural satellite is named Weywot.
Orcus is a plutino, which means it is in a 2:3 resonance with Neptune, just like Pluto. It was discovered in 2004, though the earliest precovery images can be traced back to 1951. It is located roughly 39 AU from the Sun and completes an orbit in 245 years.
Orcus has a large moon called Vanth. This natural satellite is half the size of Orcus. In fact, it is the third-largest moon beyond Neptune. The first two are Charon (Pluto) and Dysnomia (Eris).
Arrokoth is KBO that looks like a snowman. It is located about 45 AU from the Sun. This object is important in the history of space exploration. In 2019, NASA’s New Horizons spacecraft flew by it. So far, it is the farthest and the oldest object in the solar system that was visited by a man-made spacecraft.
Arrokoth is a contact binary. That means it is composed of two bodies that have joined together. The two planetesimals that make it up are 21 km (13 mi) and 15 km (9 mi) in diameter.
Arrokoth was discovered in 2014. At the time, much was still unknown about it. Also, it did not have an official name yet so it was nicknamed “Ultima Thule.” It became the first New Horizons target after its successful flyby on Pluto.
The Ever-Elusive Planet Nine
The objects in the Kuiper belt have strange orbits. While some are stable, some are also affected by Neptune if they get too close to the planet. The mystery, however, does not end there.
Extreme trans-Neptunian objects are bodies that are far beyond Neptune. They are not known to be affected by any known giant planets. However, the orbits of these objects are clustered in a way as if they are influenced by an undiscovered planet. This hypothetical planet is called Planet Nine.
Scientists have not seen Planet Nine. However, they inferred its existence because of the gravitational effects on other KBOs. This hypothetical planet is predicted to be 5 to 10 times as massive as Earth. Also, it is believed to be 400 to 800 times as far from the Sun.
So far, sky surveys have not detected Planet Nine. Its existence is not yet proven, but also not disproven.
Explorations and Missions
Artist’s impression of NASA’s New Horizons spacecraft, en route to a January 2019 encounter with Kuiper Belt object 2014 MU69.Credits: NASA/JHUAPL/SwRI
NASA’s New Horizons spacecraft marked the first mission to explore the Kuiper belt. It launched on January 19, 2006, and made a flyby of Pluto on July 14, 2015. After that, it continued to examine the Kuiper belt environment.
The spacecraft revealed three potential targets after Pluto. The first target, nicknamed “Ultima Thule” was chosen on August 26, 2015. It made a flyby on this object in 2019. Now, this small body is known as 486958 Arrokoth.
No other missions to the Kuiper belt have followed the New Horizons. Though, many experts have conducted different studies to calculate concept missions. Some of the considered targets are 50000 Quaoar, 28978 Ixion, and 47171 Lempo, among others.
More Interesting Kuiper Belt Facts
- We had the first glimpse of the Kuiper belt in 1930 when Pluto was discovered. However, it was only in 1992 when, 1992 QB1, the second Kuiper belt object was known. That is 62 years apart!
- The term “cubewano” for classical KBOs was derived from 1992 QB1. Since it was not assigned a name until 2018, similar objects discovered later were called QB1-o’s” (cubewanos) after it.
- Pluto is called the “King of the Kuiper Belt.” It earned that title because it is the largest object in the Kuiper belt.
- Pluto was still classified as a planet when the New Horizons spacecraft took off on January 19, 2006. By the time, it was the only unexplored planet in the Solar System. On August 24 of the same year, the International Astronomical Union (IAU) announced its new classification.
- Some of the dwarf planets in the Kuiper belt have atmospheres. However, these are very thin. When the highly elliptical orbit takes these objects very far from the Sun, their thin atmosphere collapses.
- The dwarf planets in the Kuiper have their own sets of moons. Pluto has 5 moons, Haumea has 2, while Makemake has 1 provisional moon. Also, Haumea even has a ring around it!
- There are structures similar to the Kuiper belt around at least nine other stars according to astronomers. The Hubble Space Telescope observed disks around the stars HD 138664 in the Lupus constellation and HD 53143 in the Carina constellation.
Main part of the Kuiper belt:
View from a KBO: